Tell Explorer to display a folder in thumbnail view and it begins to generate thumbnails for every supported file. These are then saved in various thumbnail caches, thumbcache_*.db and iconcache_*.db, so that they don't have to be created again.
What's interesting about these thumbnails is that they can live on for quite some time, even if you delete the source files. And so if you've accidentally deleted some important images yourself - or want to see what another user has been viewing - then browsing the cache may reveal all.
Thumbcache Viewer is a tiny portable tool (53KB) which can display the contents of any thumbcache_*.db and iconcache_*.db file. Just launch it, click File > Open and choose your preferred cache to view its details in a table. (Start by looking for the largest thumbcache*.db files in \Users\<UserName>\AppData\Local\Microsoft\Windows.)
The table is horribly cryptic, packed with low-level details, checksums, hashes and more. It doesn't even list the original file name.
Fortunately, if you click an entry in the table, a separate window opens to display it. All you then really need to know is that the most recent images have the highest numbers, and are at the bottom of the table. Click on one of these, and use the cursor keys to scroll up; the preview window updates automatically to show the image, and you can save the current option - or everything in the cache - in a couple of clicks.
It won't find every lost image. And even if you do turn up something interesting, the resolution may be too low to be useful. But Thumbcache Viewer turned up some surprisingly old 1024x1024 thumbnails on our test PC, and it's worth keeping a copy around for emergencies.